docs: def plot_tree(ax, tree, plane='xy',
                diameter_scale=_DIAMETER_SCALE, linewidth=_LINEWIDTH,
                color=None, alpha=_ALPHA):
      '''Plots a 2d figure of the tree's segments
  
      Args:
          ax(matplotlib axes): on what to plot
          tree(neurom.core.Tree or neurom.core.Neurite): plotted tree
          plane(str): Any pair of 'xyz'
          diameter_scale(float): Scale factor multiplied with segment diameters before plotting
          linewidth(float): all segments are plotted with this width, but only if diameter_scale=None
          color(str or None): Color of plotted values, None corresponds to default choice
          alpha(float): Transparency of plotted values
  
      Note:
          If the tree contains one single point the plot will be empty
          since no segments can be constructed.
      '''
      plane0, plane1 = _plane2col(plane)
      segs = [((s[0][plane0], s[0][plane1]),
               (s[1][plane0], s[1][plane1]))
              for s in iter_segments(tree)]
  
      linewidth = _get_linewidth(tree, diameter_scale=diameter_scale, linewidth=linewidth)
      color = _get_color(color, tree.type)
  
      collection = LineCollection(segs, color=color, linewidth=linewidth, alpha=alpha)
      ax.add_collection(collection)